Synthesis, crystal structures, and solution properties of N-methylene(phenyl)phosphinic acid derivatives of cyclen and cyclam

Two phenylphosphinic acid derivatives of tetraaza macrocycles, (1,4,7,10-te traazacyclododecanetetrayl)tetrakis (methylene)tetrakis(phenylphosphinic ac id)l (H4L1) and (1,4,8,11-tetraazacyclotetradecanetetrayl)tetrakis(methyl e ne)tetrakis (phenylphosphinic acid)] (H4L2) were synthesised and the crysta l structures of H4L2 and its bis(1-adamantylammonium) salt were determined. In both the structures, the ring conformation is virtually the same and is stabilised by hydrogen bonds. Protonation constants of the acids and stabi lity constants of their Zn2+ and Cd2+ complexes were determined pH-metrical ly at 25 degrees C and at an ionic strength of 0.1 mol dm(-3) (KNO3). The p K(A) values found for both the-compounds are lower than those for their car boxylic analogues and correspond to the expected electron-withdrawing abili ty of the -P(Ph)O2H moiety. The value pK(1) of the last dissociation consta nt of H4L1 is surprisingly high. The complexes of Zn2+ and Cd2+ with (L-1)4 - and (L-2)(4-) are less stable than those with other similar ligands, whic h is in accordance with lower overall basicity of H4L1 and H4L2. A pronounc ed drop in the stability of the complexes of H4L2 can be explained by parti al coordination of the ligand to the metal ions. The solution properties of the ligands were also investigated by P-31-NMR spectroscopy. The spectra o bserved suggest stable ton the NMR time scale) conformations of the protona ted ligands in solution. A comparison of NMR spectra in solution and CP/MAS -NMR spectra indicates that the structures of the conformers roughly corres pond to the structures observed in the solid state. The conformations are s tabilised by intramolecular hydrogen bonds and by hydrophobic interactions of phenylphosphinic moieties. The conformers are stable even at 90 degrees C, but not in the presence of complexing metal ions.